PROCESS FOR ATOMISING ELECTROLYTES AND THE CHEMICAL ANALYSIS THEREOF

Abstract: The present invention relates to a one-step process useful in the chemical analysis for atomising electrolyte solutions or fluids with electric conductivity, and a multielement direct analysis method of the components of such fluids, which is also operable in a monitoring mode. In this analysis method, the one-step atomisation of the fluid is followed by optical and/or mass spectroscopy determination of the sample components. The atomisation process is characterised by generating an electric discharge of 1000 to 5000 V/cm on the surface of said electrolyte between the fluid as cathode and the anode positioned in the gas atmosphere above the said fluid.

Description: Process for atomising electrolytes and the chemical analysis thereof TECHNICAL FIELD The present invention relates to a one-step process useful in the chemical analysis for atomising electrolyte solutions or fluids with electric conductivity, and a mult¬ ielement direct analysis method of the components of such fluids, which is also operable in a monitoring mode. In this analysis method, the one-step atomisation of the fluid is followed by optical and/or mass spectroscopy determination o the sample components. BACKGROUND ART In many fields of optical spectroscopy (atomic absor tion or atomic emission spectrophotometry, atomic fluores- cence) and mass spectroscopy the precondition of measurement is the existence of the components in an atomic vapour state The term: "atomisation" defines a complex process wherein an atomic aerosol is obtained from the sample by one or multist energy transfer. In the followings, the term: "atomisation" used in this sense. According to the prior art, most of the atomisation processes used for analytical purposes prepares first a coarse aerosol by pneumatic vaporisation of the electrolyte solution, which is then transferred into properly selected chemical flame of high temperature or plasma flame generated by high frequency /Boumans (Ed) : Theory of Atomic Emission Spectroscopy/. In these processes the vaporisation step, due to the small sample requirement, is performed by capillary blotting or blow-slot method and accordingly, they can be us in the analysis of solutions which are predominantly clear and/or contain little amount of floating material, which wou mean the necessity of inserting a pre-filtering step before vaporisation in many practical processes. The mass flow of atomisation, i.e. the amount of electrolyte atomised in a time unit is determined by the atomisation velocity, thus, regular cleaning of the vaporisation system and permanent control of the flow and pressure conditions is necessary. Another drawback of such processes is that atomisation by chemical flames needs a permanent and strict control due to the extraordinarily flammable and explosive gases (acetylene dinitrogen oxide, oxygen) used therein. This disadvantage do not appear in the high frequency-generated plasma flame atom sation, but the high frequency unit is of complicated struc¬ ture and high energy consumption. Another method for solution atomisation used for ana- lytical purposes is the electro-thermal heating of a precise determined quantity of electrolyte by programmed steps of evaporation, drying and vaporisation, the so-called graphite furnace atomisation. This method provides an advantageous hi atom vapour concentration, however, the process is complicat and consists of multiple steps, automatisation of sample injection is difficult to accomplish, and continuous working mode is impossible due to permanent contamination of the graphite furnace by the non-evaporating parts of the electro lytes. In the optical spectroscopy, other methods for quasi- continuous atomisation and analysis connected therewith in elude "weeping carbon electrode" method (Feldman,C: Anal.Chem. 21: 1041 /1949/) , "capillary electrode carbon arc method (Nalimov, V.V. : Zav.Lab. 23: 1351 /1955/, Zink, T.H. Appl. Spectr. 13: 94 /1959/) , rotating disc electrode spark excitation (Pierucci.M et al: Nuovo Cimento 17: 275 /1931/) and feeding electrolyte in the spark gap (Twyman.F. et al: Proc.Roy.Soc. 133: 72 /1931/) . These methods have never bee widely used due to their common disadvantage that during atomisation, irreversible deposits are formed on the solid electrodes, and accordingly, they cannot be used in continu technologies. Summarised, the known analytical methods based on analytical chemical atomisation processes of electrolyte- containing fluids (atomic absorption, atomic emission, atomi fluorescence, mass spectroscopy) require a very thorough sample preparation, (filtration, recovery, dissolving) , precise solution administration, permanent control (explosio risk in technology) and they are not suitable for continuous control-free (monitoring) system. Accordingly, the present application aims to provide one-step method for atomising electrolyte solutions and an atomic spectroscopy based thereon for a multielement analysi which eliminate the above disadvantages. DISCLOSURE OF THE INVENTION The invention is based on the recognition, that atomi sation of an electrolyte may be carried out by generating an electric gas discharge of 1000 to 5000 V/cm intensity on the surface of the electrolyte between the fluid as cathode and anode arranged in the atmosphere above the electrolyte. It has been found that as an embodiment, the following method i suitable for one-step direct multielement analysis: The pH-value of the fluids is optionally adjusted to acidic, preferably in the interval of 1,0 to 2,0, a part of the fluid is atomised by generating electric gas discharge o the surface of fluid between the electrolyte as cathode and the anode arranged in the gas atmosphere above the electro¬ lyte, and the constitution of the sample is determined by methods known per se, by optical spectrometry based on light emission created by excitation of the atomic vapours obtaine from the components, or by direct mass spectrometric analysi of the atomic vapours. It has also been found that the presence of an extrem ly high amount of alkali earth metal in the sample (10 to 10 g/1) can substitute the acidifying step, and by using the above electrode arrangement and field strength interval, bot the atomisation and the analysis connected therewith can be carried out. According to a preferred embodiment of the invention the atomisation of the electrolytes can be carried out by generating a direct current gas discharge of a field strengt from 1000 to 5000 V/cm on the fluid surface between the elec trolyte as cathode and the anode positioned in the vapour atmosphere above the electrolyte. The field strength can be controlled by the voltageand the distance between the elec trodes. The atomisation mass flow, i.e. the quantity of the sample atomised and entering the plasma in a time unit reach a defined, constant value by adjusting the field strength an the current rate to a constant value, and thus, sample admin istration becomes superfluous. In the gas discharge process, only atomisation of the cathode takes place; the anode serves for conducting away th electrons formed in the discharging process (G.Francis: Glow Discharges at Low Pressures, Handbuch der Physik Vol.XXII, Springer V. Berlin 1956) . Thus the material of anode does no disturb the process provided that it is sufficiently heat- resistant. SUMMARY OF THE DRAWINGS Fig 1. shows a possible embodiment of the atomisation. Fig 2. shows a preferred embodiment of the pulse generation and time resolution measurement BEST MODE OF CARRYING OUT THE INVENTION A possible embodiment of the atomisation is shown on Fig.l. In the discharge cell made of a non-conducting materi (glass) (1) a heat-resistant anode electrode (Tungsten) (2) inserted to a distance of 3 to 6 mm above the surface of the electrolyte cathode (5) . The electrolyte cathode (5) is in electric contact with the electrode (3) via the diaphragm (4 The electrode (3) is placed in the pace of the effluent elec trolyte (9) . The electrode (3) is connected with the negativ pole of a high voltage power supply, while the electrode (2) is connected to the positive pole thereof. The light emissio of the discharge plasma (7) forming in the gas region (6) is detected by an optical spectrometer (12) directed in the observation direction (10) . The sample is fed into the cath¬ ode space (5) through the connection (8) . The atomisation can be carried out by single sample injection or during flow of the fluid containing the electro lyte. Flow of the solution containing the electrolyte in th gas discharge pole space is especially preferred as the fl of fluid can also be used for heat transfer. In the single sample injection mode, cooling can be accomplished e.g. in a cell with coolant jacket. The preparatory process of the electrolyte solutions for analysis occasionally needs a pretreatment step. If the sample is a dilute solution, pretreatment constitutes adjust ment of the pH value, which is accomplished suitably with mineral acids , preferably to a pH value of 1,0 to 2,0. Con¬ trol of the adjustment in a static or dynamic system may be carried out in methods known per se. e.g. by pH measurement. Should the sample contain the alkali and alkali earth metal salts in a higher concentration, e.g. 10 to 100 g/1, acidification may be omitted. The gas plasma atomised according to the invention excites a part of the components of the atomic vapour to lig emission. This light radiation forms the basis of the optica spectroscopy analysis. Accordingly, the intensity of light radiation emitted by the excited components of electrolyte solutions induced by one-step atomisation, such as metal atoms, can be determined by direct optical spectroscopy meth ods, which provides signs having proportional intensity with the concentration of a component at a wave length character¬ istic to each component. The non-excited components can be analysed by directing them out the atomised gas atmosphere, e.g. by vacuum, and by mass spectroscopy methods known per s The interfering background radiations known from opti cal emission spectroscopy (e.g. the bands of radicals formed from the solvent and the components of the gas atmosphere, resp.), are also present in the emission spectrum of the plasma atomised according to the invention, and, in certain ranges, they substantially reduce the accuracy of the analyt cal measurement by increasing the background noise. It has also be found that the band radiation of the atomising gas plasma molecules and radicals according to the invention can substantially be reduced periodically by using additive current impulse. The direction of the applicable current impulse is identical with that of the discharge sup¬ porting current, its strength is 2 to 10 times higher, the term is 100 to 1000 microseconds, the upper limit of the dut cycle is about 20%, i.e. an 1 to 4 pulse:interval ratio. The intensity of the light emitted should suitably be determined in the second half of the current pulse, or in th term 10 to 100 microseconds after the end of the current pulse, wherein the intensity of the background signal de¬ creased to a low level, or, in the latter case approached or reached the dark current level of the detector. A preferred embodiment of the pulse generation and ti resolved measurement is shown on Figure 2. A section of the field current increased during one pulse is illustrated on Figure 2/a. On the basic discharge current (1) a current pul (2) of 100 to 1000 microseconds is superimposed. Figure 2/b shows the concurrently registrable detector signal function. Curve 3 corresponds to the intensity of the band background radiation, curve 5 is a typical example of the atomic line intensity during the current pulse and closely before and after thereof. The line intensity (5) measured during the in erval (6) , was improved to 2 to 5-fold of the sign/backgroun ratio (compared to background 3) and 2 to 10-fold of the pulseless direct current excitation. The term (7) is a back¬ ground decay period after the current impulse (2) , wherein t intensity of plasma emission might decrease almost to zero. The pulse excitation and time resolved measurement method according to the invention can be evaluated with a calibration curve taken between the signal intensity during the time intervals 6 or 7 on a wavelength characteristic to the target component and the concentration of the said comp nent. The possibility of continuous sample feeding consti- tutes one of the greatest advantages of the present inventi as the method can be used in complex industrial and environ mental monitoring systems. A further great advantage of the multielement analysis according to the invention is that th known processes are substantially simplified by eliminating the preparation of samples through controlled mass flow dir atomisation, i.e., without pulverisation, drying and therma vaporisation. In the process according to the invention the atomis tion mass flow is 1 to 2 orders of magnitude smaller than t of known atomisation processes, and accordingly, the sensit ity of the method is at least one order of magnitude less t that of the laboratory atomic spectroscopy methods. However, this sensitivity of the analytical method according to the invention is suitable in most of the industrial, environmen tal, etc., analytical processes. The progressive effects of the invention can be summ rised as follows: - one-step, direct atomisation by atomisation mass flow co trolled by the field strength and discharge current strengt - the atomisation takes place on the free fluid surface of sample flow, a permanently recovering surface is formed, i. the sample preparatory line needs not more than an eventual crude filtration, - a steady state is formed by permanent ventilation of the recovering fluid surface and the gas space, as well as the heat removal, i.e. there is no deposition surface, and atom sation process can be maintained continuously as well.

Claims: CLAIMS 1. Process for atomising an electrolyte solution, characterised by generating an electric discharge of 1000 t 5000 V/cm on the surface of said electrolyte between the fl as cathode and the anode positioned in the gas atmosphere above the said fluid. 2. A method for the multielement analysis of electro lyte solutions, characterised by optionally acidifying the said solutions, atomising a part of the fluid by generating electric discharge of 1000 to 5000 V/cm on the surface of said electrolyte between the fluid as cathode and the anode positioned in the gas atmosphere above the said fluid, and subsequently determining the constitution of the sample by optical spectrometry based on the light emission formed by excitation of the atomic vapours of components or by direct mass spectroscopy analysis of the atomic vapours by methods known per se. 3. The method according to claim 2 characterised by adjusting the pH value of the solution by the addition of a acid to a value of 0,5 to 3, preferably between 1 and 2, in case the electrolyte solution contains the alkali and alkal earth metal ions in a concentration of less than 2 g/1. 4. The method according to claim 2 characterised by adjusting the pH value of the solution between 3 and 5, in case the electrolyte contains 2 to 10 g/1 alkali or alkali earth metal ions. 5. The method according to any of claims 2 to 4 cha¬ racterised by generating the gas discharge by current pulse superimposed on direct current wherein the pulses are 2 to times higher, have a term of 100 to 1000 microseconds and a duty cycle of maximum 20%, and determining the intensity of light emission of the components in the second half of the current impulses. 6. The method according to claim 5 characterised by determining the light emission of the components in the ter 10 to 100 microseconds after the end of the current impulse

Inventors: CSERFALVI TAMAS, MEZEI PAL, CSERFALVI, TAMAS, MEZEI, PAL
Applicants: AQUA CONCORDE KFT, CSERFALVI TAMAS, MEZEI PAL, AQUA-CONCORDE KFT., CSERFALVI, TAMAS, MEZEI, PAL
Application year: 1994
Main classification: G01N21
IPC codes: G01N1, G01N21, G01N27